1. Field of the Invention
The present invention relates to a demodulator for demodulating a phase modulation signal in an optical fiber communication system, and an optical transceiver using the same (an optical communication module).
2. Description of the Related Art
In the field of optical communications, a simplest method whereby intensity modulation is performed as signal modulation and light intensity is directly converted to an electric signal using a photodetector as demodulation has long been used. However, in order to support a high bit rate exceeding 40 Gb/s in recent years, a system that performs phase modulation as signal modulation is attracting attention. As demodulation methods of a phase modulation signal, there are the following two ways: a method whereby a light that was signal modulated and transmitted is demodulated by making it interfere with a light from a local oscillator light provided on a receiver side (a coherent method); and a method whereby a light being signal modulated is branched, the branched beams are multiplexed with a timing between them shifted by one bit of signal modulation to cause interference with each other, and the signal is demodulated by converting a shift of phase into a light intensity signal (a differential phase-shift keying method). The differential phase shift keying method is called Differential Binary Phase Shift Keying (DBPSK or DPSK) or Differential Quadrature Phase Shift Keying (DQPSK), etc depending on the number of phases to be modulated. In the coherent method, for example, a case of phase quadrature modulation is called QPSK (Quadrature Phase Shift Keying). Both of the above-mentioned methods can be used with a polarization diversity method for modulating two polarization components of the light independently, and this combination makes it possible to increase information quantity twofold. Especially in the coherent system, a system called DP-QPSK (Dual-Polarization QPSK) that combines the polarization diversity system and QPSK is also being investigated.
The demodulation method in DQPSK will be explained using FIG. 1 that is a block diagram of the demodulator. A modulated light 100 modulated by the differential phase shift keying is first branched into a branched light 102 and a branched light 103 by a branching element 101 like a half beam splitter. The branched light 102 is further two-way branched by a branching element 104, which is configured so that one of these may be added a optical path length of one symbol of signal modulation (for example, when a signal modulation frequency is 20 GHz, about 14 mm) and a light path difference between two branched lights of the branching element 104 may be an integer times of the light wavelength (namely, the phase difference is zero). Then, the two branched lights are again multiplexed with a multiplexing element 106 such as a half beam splitter, and two interference lights are generated. An output signal is acquired by detecting an intensity difference of these interference lights with a differential detector 107 consisting of a balanced photodetector and a trans-impedance amplifier. A channel of the output signal acquired from these interference lights of a phase difference zero is called an I channel. On the other hand, the branched light 103 from the branching element 101 is two-way branched by a branching element 108 similarly with the branched light 102, one of the branched lights being given a delay of one symbol by a delay part 109, and the two branched lights are multiplexed again by a multiplexing element 110 to generate the two interference lights. However, the two branched lights are configured so that the phase difference may become 90-deg. These interference lights are detected by a detector 111 similarly with the I channel and an output signal is obtained. The output channel obtained from the interference light of this 90-deg phase difference is called a Q channel.
FIG. 2 that is a block diagram of the demodulator shows a demodulation method in DP-QPSK. A modulated light 200 (hereinafter referred to as a signal light) is separated into two polarization components by a polarization separating element 201, such as a polarization beam splitter. Similarly, a local oscillator light 202 prepared on a receiving side is also two-way branched by a branching element 203. Here, the branched lights of the signal light and the local oscillator light are inputted to one of optical 90-deg hybrids 204, 205, where the interference light of the signal light and the local oscillator light is generated. At this time, the signal light and the local oscillator light are each branched by an optical branching element inside the optical 90-deg hybrid, and two kinds of multiplexing are performed at a phase relationship where the lights are mutually different in phase by 90-deg. Here, since the two interference lights generated by a single time of multiplexing are detected by any one of the balanced detectors 206, 207, 208, and 209 to output an electric signal corresponding to the intensity difference, output signals of the I channel and the Q channel are generated for one polarization component similarly with the demodulator of DQPSK.
As described above, for the demodulation of the phase modulation signal, an interferometer type optical system for multiplexing multiple lights and making them interfere is used. Moreover, as was described in examples of DQPSK and DP-QPSK, since outputs of two channels are required for demodulation of a quadrature phase modulation signal, two kinds of multiplexing at phase relations mutually different by 90-deg is performed to generate the interference lights. As such a mode for carrying out the invention, there are known Japanese Unexamined Patent Application Publication No. 2008-278249 (corresponding U.S. Ser. No. 12/104,056), Japanese Unexamined Patent Application Publication No. 2006-287493 (corresponding U.S. Ser. No. 11/391,414), Japanese Unexamined Patent Application Publication No. 2007-306371 (corresponding US 2007/0264029), Japanese Unexamined Patent Application Publication No. 2008-17445 (corresponding U.S. Ser. No. 11/479,920), Japanese Patent No. 4170298 (corresponding U.S. Ser. No. 11/117,429), and Japanese Unexamined Patent Application Publication No. 2006-270909 (corresponding U.S. Ser. No. 12/656,413).
As modes of the interferometer for carrying out the invention described above, there are a mode mainly using a planar waveguide circuit and a mode using a free space optical system with bulk optical elements, and the latter is characterized in being low cost compared to the former.